1. Technical Field
This invention relates to lithium ion batteries, and more particularly to a method of making electrodes (i.e. anodes and cathodes) therefor.
2. Description of the Related Art
Lithium ion batteries of the so-called xe2x80x9crocking chairxe2x80x9d type are known in the art and comprise a lithium-intercalateable anode, a lithium-intercalateable cathode and a lithium-ion-conductive electrolyte sandwiched therebetween, as seen generally by reference to U.S. Pat. No. 5,196,279 to Tarascon. One particular variant of such battery is the so-called xe2x80x9clithium polymerxe2x80x9d battery wherein (1) the electrodes (i.e. anode and cathode) contain lithium-intercalateable particles bound together in a porous polymer matrix, impregnated with electrolyte, and (2) a porous polymeric membrane/separator, impregnated with electrolyte, lies interjacent the electrodes.
It is known to fabricate lithium-polymer cells by sandwiching a thin dry film of the separator/membrane material between a thin dry film of anode material and a thin dry film of cathode material and forming a laminate thereof by bonding the several films together under heat and pressure. Current collecting grids may be pressed into the anode and cathode materials at the same time or in a separate operation. However, this approach involves many steps, which increase fabrication cost and complexity. Moreover, achieving consistent and enduring lamination has been an ongoing problem in the manufacture of lithium polymer batteries. Delamination of one or more layers may result in an inoperative battery.
Other approaches have been taken in the art. U.S. Pat. No. 5,296,318 to Gozdz et al. disclose a process for making a lithium polymer cell by a process wherein (1) a first electrode film is cast wet and dried on a first current collector defined by aluminum collector foil, (2) a separator/membrane film is cast wet and dried atop the first electrode film, (3) a second electrode film is cast wet and dried atop the separator/membrane, and (4) a second current collector applied to the second electrode film. However, the approach is not effective for mass production inasmuch as the process produces incomplete and/or unenduring contact between layers and components thereof. This is more particularly true for the above-mentioned lamination approach. The foregoing results in lower production efficiency, increased scrap rate (due to higher than acceptable resistances), and, accordingly, higher costs.
There is therefore a need to provide an improved process for fabricating composite electrodes for use in lithium ion batteries or cells that minimizes or eliminates one or more of the problems as set forth above.
Manufacturing complexity, cost, and scrap rate can be reduced, production rates increased, and better contact between the grid and the electrode material achieved by a process according to the present invention.
The invention involves applying a wet electrode film-forming slurry to a substrate such as Mylar and bedding in one face of a current collecting grid. The current collecting grid needs to be coated from both sides, it requires a second coating of the same slurry. The solvent from the slurry dissolves at least some of the dry electrode film causing the two films to coalesce and adhere to the grid in a manner superior to that achieved by heat/pressure bonding. The solvent is subsequently removed leaving the grid buried in the dried films. Moreover, less production scrap is generated compared to heat/pressure bonding processes that tend to entrap air between the films and interfere with good film-to-film and film-to-grid contact/bonding. Electrodes having poor film-to-film and film-to-grid bonding have unacceptably high resistances, poor capacity, and poor cycling and accordingly must be scrapped.
In a preferred embodiment where the current collecting grid needs to be coated from both sides, the process for making lithium-intercalateable electrodes for a lithium polymer battery according to the invention involves the steps of: applying a film onto a first face of an electrically conductive grid, which film comprises lithium-intercalateable particles dispersed throughout a mixture of a polymeric binder and a plasticizer for the binder; applying a film-forming slurry onto a second face of the grid, which slurry comprises lithium-intercalateable particles dispersed throughout a mixture of polymeric binder, a plasticizer for the binder and a solvent for the binder; removing the solvent from the slurry so as to leave a dry film of particles, binder and plasticizer adhering to both sides of the grid; removing the plasticizer from the film so as to leave a network of pores therein; and backfilling the pores with a lithium-ion-conductive electrolyte.
According to another embodiment of the invention, optional steps includes applying a wet electrode film-forming slurry to a first face of a carrier strip preferably comprising Mylar material, and drying the same to form a first dry film (i.e., solvent removed); applying a current collecting grid to the first dry film; and then applying a wet film (i.e., still containing solvent) to the second face of the grid such that the solvent from the second film serves to dissolve at least a portion of the first film causing the first and second films to coalesce with each other while bonding to the grid.
According to another variant of the invention, both films will be applied to the grid wet (i.e., no intermediate drying). Drying may be effected by heating, vacuum, forced air or otherwise, and the electrode may be compressed following drying. Preferably, drying will be by heating and compressing will be done while the electrode is still warm from the drying operation.